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Image courtesy of Judith Lean
Though the sun's brightness was once thought to be constant, NASA has launched a series of satellite instruments that have helped show it actually fluctuates in conjunction with cycles of solar activity.

With a new sun-watching instrument called the Total Irradiance Monitor (TIM) scheduled to launch on NASA's Glory satellite in November, we spoke with Judith Lean, a member of the Glory science team and solar physicist at the United States Naval Research Laboratory, about solar cycles and what scientists have learned about solar variability in the last three decades.

What is a solar cycle and how long does it last?
For more than a century, people have noticed that sunspots become more and less frequent on an 11-year-cycle. That’s the main solar cycle we look at. The 11-year-cycle is really part of a 22-year-cycle of the sun’s magnetic field polarity. The changes are driven by something called the solar dynamo, a process that generates and alters the strength of the magnetic field erupting onto the sun's surface. It's the sun’s magnetic field that produces sunspots as it moves up through the sun's surface.

How much does the brightness of the sun change throughout the cycle?
It's a small amount. Total solar irradiance typically increases by about 0.1 percent during periods of high activity. However, certain wavelengths of sunlight—such as ultraviolet—vary more.

What causes irradiance to change?
It's really the balance of sunspots, which are cooler dark areas of the sun, and faculae, bright areas that appear near sunspots. The faculae overwhelm the sunspots, so the sun is actually brighter when there are more sunspots.

Can changes in the sun affect our climate?
If it wasn’t for the sun, we wouldn’t have a climate. The sun provides the energy to drive our climate, and even small changes in the sun's output can have a direct impact on Earth. There are two ways irradiance changes can alter climate: One is the direct effect from altering the amount of radiation reaching Earth. The second is that solar variability can affect ozone production, which can in turn affect the climate.

Does the 0.1 percent change in irradiance affect Earth's climate much?
Solar irradiance changes are likely connected to dynamic aspects of climate—things like the coupling of the atmosphere and ocean—El Niño being one example—or aspects of atmospheric circulation, such as the Hadley cells that dominate in the tropics.
But we've done a great deal of modeling, and the sun doesn't explain the global warming that's occurred over the last century. We think changes in irradiance account for about 10 percent global warming at most. Of course, there are also longer cycles that may have an impact on climate, but our understanding of them is limited.

There is disagreement about whether the last three cycles have gotten successively brighter. Has that been resolved?
No, it hasn't. The best understanding is that irradiance cycles have been about the same in the last three cycles, but one group reports an increasing trend whereas another group says that current levels are now the lowest of the entire 30-year record. I believe these differences are due to instrumental effects, but we really need continual, highly accurate, and stable long-term measurements to resolve this. The radiometer aboard Glory—the Total Irradiance Monitor (TIM)--will be a big step, quite an exciting advance.

What part of the 11-year cycle will Glory observe?
Glory is going is to observe during the ascending phase of the cycle. The ascending phase is relatively rapid, so we should get to the peak in about three years. Then there will be about two years or more when solar activity is high and stays high. About five years from now, activity will start to come down again so that by, say, 2019 we will be at low levels again.

What do you hope Glory will find?
The Glory TIM has been calibrated more rigorously than previous instruments, so it should help a lot in getting the absolute brightness of the sun. In addition to recording the ever-changing irradiance levels, it should measure irradiance precisely enough that will make it feasible to determine whether solar irradiance is stable or changing, if the measurements continue long enough into the future.

Are there aspects of the solar variability that TIM won't measure?
Yes. The Glory TIM looks at overall irradiance, but it doesn't measure how specific parts of the spectrum—the ultraviolet, visible, or infrared—are changing. Some of the largest changes actually happen at the shortest wavelengths, so it's extremely important that we look at the spectrum. There's an instrument related to TIM called the Solar Irradiance Monitor (SIM) aboard the SORCE satellite that lets us see how individual parts of the spectrum vary, and it's also critical.

The sun has been exceptionally quiet in recent years. Are we entering a prolonged solar minimum?
There was a period from mid-2008 to mid-2009 when the sun was without sunspots for many days. It was probably the quietest period we've seen since the first total solar irradiance measurements. But we didn't go into a prolonged minimum because the sun still had a few active regions – not sunspots, but small bright faculae regions -- and we could see the irradiance continue to fluctuate throughout this very quiet period. Now there are more dark sunspots and more bright faculae on the sun’s surface, so activity is ramping up and a new cycle--solar cycle 24--has started.